All natural water contains dissolved chemicals. Some of these chemicals may precipitate on hot surfaces of heaters, forming scale. Mainly, scale contains calcium salts of sulfates, carbonates, oxides, etc. Relatively low concentrations of magnesium, aluminum and iron salts can be also found in scale.
A typical electric heater for heating water and other liquids comprises a heating unit or, more specifically, electrical resistance heating material which converts an electric current flowing through the material into heat. This unit is usually enveloped by a heat conducting sheath comprising one or more layers of electrically insulating compound, which are capable of a reasonably high heat transfer from the heating unit to the liquid. On the other hand, the scale that is formed on the surface of the sheath has poor thermal conductivity. Accordingly, its accumulation may cause the unit to overheat and fail to operate. In addition, mass of the scale may physically deform the heater thus also causing its failure. Finally, scale tends to exfoliate from the heater surface into heated liquid, thus contaminating the liquid.
Various solutions have been proposed to inhibit scale formation on heaters. Some of such techniques are disclosed for example in the following publications: U.S. Pat. No. 7,299,742 to Meineke; U.S. Pat. No. 5,774,627 to Jackson; U.S. Pat. No. 6,744,978 to Tweedy et al.; U.S. Pat. No. 5,586,214 to Eckman; U.S. Pat. No. 6,205,291 to Hughes et al.; U.S. Pat. No. 6,571,865 to Shi et al; and U.S. Pat. No. 6,909,841 to Linow et al.
In particular, U.S. Pat. No. 7,299,742 discloses an apparatus for preparing hot beverages that includes a boiler and a device for inhibiting scale. That device comprises at least one ultrasound transmitter located at the boiler, inside the boiler or in the region of the boiler. The ultrasound transmitter is operatively coupled to the boiler and excites it to oscillate with its natural frequency.
U.S. Pat. No. 6,744,978 describes heating elements and methods for their fabrication and use. The heating elements include a resistance heating material and an electrically insulating, substantially water impervious sheath disposed over the resistance heating material to form an active element portion having an envelope of about 50 in3, a total wattage of at least 1000 W, and a watt density of no greater than 60 W/in2.
U.S. Pat. No. 6,571,865 describes a water heater comprising an exposed heat transfer surface with water in contact with the exposed heat transfer surface. The heat transfer surface includes a layer of tetrahedral amorphous carbon and/or diamond-like carbon, and/or a composite thereof. The heat transfer surface can be used in kettles, washing machines, dishwashers and condensers.
U.S. Pat. No. 6,205,291 discloses a scale-inhibiting water heater element. The water heater element is coated with a diamond-like coating which has low surface tension to keep scale from forming, and is thermally conductive, which helps prevent overheating. The scale-inhibiting water heater element may be fabricated, for example, by coating a standard water heater element with an amorphous silicon adhesion layer, and then applying a diamond-like coating using a pulsed-glow discharge process.
U.S. Pat. No. 5,774,627 discloses an extended life electrical heating element for a water heater that includes a coiled heating resistance wire having a uniform power output per coil turn. Where the heating resistance wire passes through the sheath at critical areas, e.g. return bends, the number of coil turns per unit length of element is reduced to reduce thermal power output per unit length of the element. The number of coil turns per unit length of element in bend areas may be reduced by simply stretching the coiled heating wire to attain the desired length of resistance wire per unit length of the element. Resistance wires of differing heat output per unit length may be combined with different degrees of stretching to achieve the desired element temperatures.
Polymeric heating elements and water heaters containing these elements are provided by U.S. Pat. No. 5,586,214 which utilize polymeric materials contacting with electric resistance heating materials and with liquid to be heated. The heating elements include an electrically conductive resistance material capable of heating liquid when energized. The winding is insulated and protected by a polymer layer integrally disposed over the resistance material.
U.S. Pat. No. 6,909,841 describes an infrared emitter element that includes at least one emitter tube made of silica glass, which has two ends; at least one electrical conductor arranged in the emitter tube as a radiation source; a cooling tube made of silica glass, which surrounds the at least one emitter tube spaced therefrom and which is connected to the at least one emitter tube directly at its ends, such that in the region of the electrical conductor at least one flow-supporting channel is formed between the at least one emitter tube and the cooling tube; and a metallic reflector. The cooling tube is completely covered with the reflector on its side facing away from the emitter tube. The infrared emitter element may be used as a flow-through heater, such as a heat exchanger, especially for high-purity fluids.
U.K. Patent Application GB2244898A describes a heating element for use in heating fluids by immersion of the element therein. The heating element is provided with a coating of a suitable plastics material capable of withstanding the elevated temperatures to which the heating element is subjected and which inhibits the deposition of scale from the heated water on that element.